1. Field of the Invention
The present invention relates to methods and apparatus for transmitting Asynchronous Hybrid Automatic Repeat request (ARQ) process identities in a wireless communication system.
2. Description of the Related Art
During data transmission, especially wireless data transmission, error inevitably occurs to decrease the quality of the transmitted data. Therefore, the data is retransmitted in order to correct the error.
Automatic Repeat-reQuest (ARQ) is an error control method for data transmission which makes use of acknowledgments and timeouts to achieve reliable data transmission. An acknowledgment is a message sent by the receiver to the transmitter to indicate that it has correctly received a data frame.
Usually, when the transmitter does not receive the acknowledgment before the timeout occurs (i.e. within a reasonable amount of time after sending the data frame), the transmitter retransmits the frame until the data within the frame is either correctly received or the error persists beyond a predetermined number of re-transmissions.
Hybrid ARQ (HARQ), is a variation of the ARQ error control method, which gives better performance than the ordinary ARQ scheme, particularly over wireless channels, at the cost of increased implementation complexity. One version HARQ is described in the IEEE 802.16e standard.
The HARQ protocol can be further classified into a synchronous HARQ protocol and an asynchronous HARQ protocol. In the synchronous HARQ protocol, the retransmissions happen at fixed time intervals and control information only needs to be transmitted along with a first subpacket transmission. The drawback of synchronous HARQ is that, however, the retransmission subpackets cannot be scheduled at preferable channel conditions because the timing of the retransmission is predetermined. Also, the modulation, coding and resource format cannot be adapted at the time of retransmission according to the prevailing channel conditions at the time of retransmission.
In the asynchronous HARQ protocol, the retransmission timing, modulation, coding and resource format can be adapted according to the prevailing channel and resource conditions at the time of retransmission. The control information, however, needs to be sent along with all the subpackets. The control information transmission along with each subpacket allows adjusting the transmission timing, modulation, coding and resources allocated.
In the third Generation Partnership Project (3GPP) Long Term Evolution (LTE) systems, a maximum of two codewords are used for transmission of two, three or four MIMO layers. In addition, an HARQ process identity is used to indicate the ID of the channel in an N-channel HARQ system. For example, a 3-bit process ID allows simultaneous operation on 8 SAW channels.
When two subpackets from two respectively corresponding codewords are transmitted using the HARQ transmission scheme, the transmission rank may change from 2 to 1 at time of retransmission. If both subpacket used a process ID of 0 (PID=0) at the first transmission in rank-2, only a single codeword can be retransmitted in rank-1. This is because a single subpacket under a single PID can be retransmitted in rank-1. The second codeword transmission has to start from the beginning at a later time. This results in loss of the previously transmitted subpacket in rank-2.
When two subpackets from two respectively corresponding codewords are transmitted using the HARQ transmission scheme, the transmission rank may also change from 1 to 2 at time of retransmission. If a first subpacket uses a process ID of 0, while a second subpacket uses a process ID of 1 at the first transmission in rank-1, the two codewords are transmitted in rank-1 in two subframes because a single codeword can be transmitted in rank-1 in a given subframe. We note that the retransmissions for the two codewords can be performed in rank-2 because the two codewords are transmitted on different hybrid ARQ processes.